Attenuation equalizer



Dec' 2, W. R. ATTENUATION EQUALIZER v Filed oct. 24, 1940 mm-/ncl PERsico/va Jg ATTORNE V Patented Dec. 2, 1941 ATTENUATION EQUaLIzEn WalterR. Lundry, Maplewood, N. J., assigner to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application october 24, 1940,serial No. 362,545

(ci. 17a-,44)

Claims.

This invention relates to wave transmission networks and moreparticularly to networks used as attenuation equalizers.

An object of the invention is to vary continuously the transmission lossof an attenuation equalizer to provide a family of similarcharacteristic curves.

' Another object is to provide such a family of curves `all of whichhave the same loss at some fixed reference frequency.

Another object is to extend the curves to the region below the axis ofzero loss.

Another object is to provide a variable attenuation equalizer having alinear loss characteristic which may be continuouslyvaried from apositive slope to a negative slope and which will have the same loss, atsome reference frequency, for all settings.

A further object of the invention is to provide an attenuation equalizerwhich will give a voltage gain when operating between a constantnonreactive input load and a high impedance output load. K

Transmission circuits often require the use of variable attenuationequalizers which may be regulated to compensate for attenuation changescaused by changes in the temperature or other weather conditions. Undersome circumstances it is desirable that the equalizer be continuouslyadjustable to provide a family of similar loss characteristics all ofwhich pass through a comlmon point. To make the equalizer more useful itis also sometimes required that the family of curves be extended intothe region of transmission gain.

The attenuation equalizer of the present invention meets all of theserequirements in a simple and eicient manner. The network comprises anumber of sections connected in parallel at their input ends and coupledattheir output ends by means of a potentiometer to a high impedanceload. The potentiometer may, for example, be of the condenser type, andthe output load may be the input circuit of a thermionic tube. 'I'heequalizer sections are preferably ofthe constant resistance type and oneor more of the sections may be ldesigned to provide a voltage gain. Theindividual sections may, for example, be so designedthat their losscharacteristics over the useful frequency range are substantiallystraight lines having different slopes, some positive and some negative,but all having the same loss at some reference frequency. Under thesecircumstances by an adjustment ofthe potentiometer there may be selectedany one of an infinite family of loss characteristics all of which arelinear and in effect pivot about a fixed point.

The nature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdrawing, in which like reference characters represent p like ior similarparts and in which:

Fig. 1 is a diagrammatic representation of a variable attenuationequalizer in accordance with the invention; Y

Fig. 2 shows the circuit of an equalizer section the loss characteristicof which will have a positive slope;

Fig. 3 shows an equalizer section the loss characteristic of which willhave a negative slope;

Fig. 4 shows the circuit of an equalizer section which will provide avoltage gain; and

Fig. 5 gives typical transmission loss characteristics obtainable withthe variable equalizer of Fig. 1.

A typical circuit of a variable attenuation equalizer in acordance with.the present invention-is shown schematically in Fig. 1, wherein 2| and22 are the input terminals and 23 and 24 are the output terminals. Thecircuit is shown in the unbalanced form and the path between terminals22 and 24 may be grounded or otherwise fixed in potential. It ispreferable that the equalizer work into a high impedance load, as, forexample, the input circuit of a thermionic tube I9, which may be thefirst tube of an amplier.

The component equalizer sections 2, 3, 5, 6 and l and the resistance pad4 are all connected in parallel at their input ends. Section 2 isconnected in tandem with section I, and the latter is terminated at itsoutput` end in a resistance 25 equalin value to the image impedance ofthe section. Sections I and 2 should have matching image impedances attheir junction. Sections 3, 5 and 6 are likewise terminated in theirimage impedances 26, 2l and 28, while section l, as explainedhereinafter, is designed to work into a high impedance load.

'Ihe output voltages of the component equalizer sections are selectivelycombined by means of the condenser potentiometer 20. The high potentialsides of the output ends of the sections I to 'I are connected,respectively, to the fixed condenser plates 3| to 3l. The movable plate30 is connected to the output terminal 23 of the equalizer as a whole.

The equalizer sections are preferably of the `constant resistance typeand may, for example,

be bridged-.T structures such as are shown in Figs. 2 and 3 and morefully described in U. S. Patent 1,606,817 to G. H. Stevenson issuedNovember 16, 1926. Section 2 may, for example, be of the type shown inFig. 2 and may have its component impedance elements so proportionedthat its loss characteristic over the useful frequency range extendingfrom f1 to f2 is substantially a straight line with a positive slope,such as curve I2 of Fig. 5. The resistance pad 8 may be connected intandem with the section 2 to increase the minimum loss at the lower endof the frequency range to some desired value. This pad may be designedto provide an impedance transformation, if desired. Equalizer section lmay also be of the type shown in Fig. 2 Aand may be designed so that,when connected in tandem with section 2 and pad 3, the entirecombination will have the transmission loss characteristic shown bycurve II ci Fig. 5. Likewise, section 3 may have the configuration ofFig. 2 and maybe proportioned so that it, together with its associatedpad 9, will have the loss characteristic shown by curve I3 of Fig. 5.

The equalizer sections 5 and 6 may have the configuration shown in Fig.3 and may be designed to have linear loss characteristics with negativeslopes. Section 5 and the associated series resistance IJ may, forexample, provide the loss of curve I5 of Fig. 5 and section 6 alone mayhave the loss shown by curve I6.

Section 'I may take the form shown in Fig. 4, described in more detailhereinafter. This section may be so designed that its losscharacteristic, as shown by curve I 'I of Fig. 5, has a negative slopeand, at its lower end, crosses the zero 1 axis and extends into theregion of negative loss, that is, transmission gain.

In the preferred form of the equalizer the pad 4 has the constant lossgiven by curve I4 of Fig.

5 and each of the sections 2, 3, 5, 6 and 'I 'in conjunction with itsassociated pad, if any, has this same loss at Ysome frequency f1 at thelower end of the frequency range covered. Section I is designed to havea minimum loss at f1 and at any other frequency has a loss equal to thedifference between curves II and I2.

Now if the movable condenser plate is placed directly opposite the fixedplate 32, for example, the loss of the equalizer as a Whole will be thatshown by curve l2. Also, if the plate 3i) is moved to a positionopposite to any of the other fixed plates 33, 34, 35, 3S or 3l the losscharacteristic will be that given, respectively, by curves I3, I4, I5,I6 or II. Furthermore, lif the plate 30 is stopped at some intermediateposition,

so that part of it is opposite one fixed plate and another part isopposite an adjacent plate, the loss characteristic Will be a curvefalling between the curves associated with the two plates. For

example, if the plate 30 overlaps both xed" plates S5 and 35 the losswill he that given by the dotted curve I8. For any of these settings theloss curve will be substantially linear and at the frequency f1 willpass through the point 39.

There is thus provided an infinite family of lini' ear curves which mayhave a positive, a zero or a negative slope and which, in effect, pivotabout the point 39. The particular characteristic obtained depends uponthe setting of the movable condenser plate 30.

Ii the plate 39 is placed opposite the fixed plate 3l the loss of theequalizer will be the sum of the losses of the sections I and 2 and thepad 8, given by curve II'. This curve does not pass through the point3%, because of the inherent minimum loss of section I, but it issubstantially linear and passes close to the pivot point. By making theplate 30 overlap portions of both of the plates 3I and 32 there may beprovided a series of curves which fall between curves II and I2.

The movable plate 30 may be adjusted manually or, if desired, it may beplaced under the automatic control of a pilot wire or a pilot channel toprovide attenuation equalization for an associated transmission circuit.

As already mentioned, equalizer section I has the property that, over atleast a portion of the frequency range it provides a transmission gain,as shown by curve I'l of Fig. 5. The design of this section will now beconsidered in greater detail. I

As shown in Fig. 4 the circuit consists of a serios impedance branch Ze,a shunt impedance branch Zb at the output end and a second shuntimpedance branch at the input end. The ratio of the input voltage Eoacross terminals 4I, 42 to the output voltage E across terminals 43, 44is given by the expression be made a constant pure resistance by addinga.

shunt branch at the input end provided either Za or Zt includes aseriesresistance, such as R in Fig. 4, which is equal to or greater thanRo. This shunt branch consists of a resistance Ro in series with ageneral impedance Ze which is the inverse, with respect to Ro, of thesum of the impedances Za and Zh reduced by the value of the resistanceRo. In equation form,

So long as Za and Zb are physical and Ris greater than Ro than Ze willbe physical.

In the above discussion it has been assumed that the equalizer sectionof Fig. 4 is to work into a load impedance which is high compared to theshunt impedance Zt. This condition is satisfied in the equalizer systemof Fig. 1 where the load is the input circuit of the thermionic tube I9.

What is claimed is:

1. An equalizer having an input impedance Ro whichis a constant pureresistance for providing a voltage gain when operating between an inputload of impedance Ro and an output load of high impedance comprising aseries branch of impedance Za, an output shunt branch of impedance Ztand an input shunt branch including a resistance of value Ro and animpedance Ze connected in series, the impedances Za and Zb having oversome portion of the useful frequency range a ratio not exceeding two,reactive components of opposite sign and resistive components the sum ofwhich is at least equal to Ru, and the impedance Ze being the inverse,with respect to Ro, of the sum of the impedances Za and Zh reduced bythe value of the resistance Re.

2. An equalizer in accordance with claim 1 in which said output shuntbranch includes a se- Which the impedance Za is of the same order ofries-connected resistor. magnitude as the impedance Zh. 5. An equalizerin accordance with claim 1 3. An equalizer in accordance with claim 1 inhaving a transmission loss characteristic which which the impedance Zahas substantially no re- 5 is substantially a straight line over saiduseful sistive component. frequency range.

4. An equalizer in accordance with claim 1 in WALTER R. LUNDRY.

